1. Field of the Invention:
This invention relates to high relative aperture Gauss type lenses of standard field angle suited to photographic cameras, video cameras, or still video cameras, and more particularly to Gauss type lenses of greatly increased relative aperture to as high as 1:1.0 in F-number with the maintenance of the regular back focal distance at an angular field of about 45.degree. , while nevertheless permitting good aberration correction to be achieved for high grade imagery.
2. Description of the Related Art
In the single lens reflex type of photographic camera or video camera, there is a quick-return mirror in the rear of the objective so that light from the objective is reflected to the finder system. As design objectives for the single lens reflex camera, therefore, the required type should meet the condition that it is easy both to obtain so long a back focal distance as to arrange the quick-return type mirror therein and to insure that high grade optical performance is obtained.
In the past, the photographic lenses of standard field angle that fulfill these requirements relatively easily have been of the so-called Gauss type. At present, this type is most often used.
To increase the speed of aperture beyond 1:1.2 in F-number with the maintenance of the regular back focal distance, however, the employment of the conventional rules of design for the Gauss type lenses will result in rapid increase of various aberrations. Particularly the off-axis halo and curvature of field are largely affected. It will, therefore, become very difficult to well correct these aberrations.
A faster Gauss type lens than 1:1.2 has been proposed in, for example, Japanese Patent Publication No. SHO 39-10178. But, because its back focal distance is as short as 0.4 times the focal length, this lens is not usable in most of the photographic cameras or in the single lens reflex cameras.